PROJECT SUMMARY Epidemiological data link history of traumatic brain injury (TBI) to increased likelihood of development of Alzheimer?s Disease (AD) and other AD-related dementias (ADRD) later in life. While recent meta-analyses estimate the overall risk of dementia attributable to TBI at 5-15%, it remains poorly understood how history of brain trauma may contribute to neurodegeneration years or even decades later. One possibility is that TBI may accelerate detrimental cellular changes occurring during normal brain ageing. A potential candidate is the autophagy-lysosomal pathway essential for degrading misfolded proteins and damaged organelles. Autophagy function declines during brain aging and more severely in neurodegenerative diseases. Thus, accelerated inhibition of autophagy-lysosomal function as compared to that observed in normal brain aging, could contribute to neurodegeneration observed in AD and other age-related dementias. We recently demonstrated that autophagy is also inhibited after TBI. This is caused by TBI-induced lysosomal defects and is associated with profound changes in lysosomal lipid composition. Lysosomal dysfunction associated with accumulation of lipofuscin and other lipid byproducts in the endo-lysosomal compartments is also observed in aged mice and is exacerbated by either drug or disease induced demyelinating episodes. Since TBI leads to myelin damage as well as more general perturbation of lipid metabolism in the brain, lipid-mediated damage could also lead to lysosomal inhibition after TBI and over time cause accelerated autophagy-lysosomal dysfunction as compared to that observed during normal aging, thus contributing to AD/ADRD. We hypothesize that perturbation of lipid homeostasis after TBI accelerates lysosomal lipid accumulation as compared to normal aging, leading to lysosomal dysfunction and autophagy defects, thus predisposing to neurodegeneration and AD/ADRD. In order to test this hypothesis, we will use HILIC-MS/MS based lipidomic analysis of brain lysosomes, MS-based lipid imaging and complementary IF/IHC and biochemical approaches to compare changes in lysosomal lipid composition and autophagy-lysosomal function in the brains of normal aging mice and mice aging after TBI. To test the causative effect of perturbed autophagy and lipid metabolism on development of AD/ADRD relevant pathological and cognitive phenotypes we will use autophagy hypomorph Becn1+/- mice and Scarb1+/- mice with hypercholesterolemia. We will also use a fly TBI model to identify additional autophagy and lipid metabolism genes linking TBI to AD/ADRD. We expect that our data will demonstrate that perturbation of lysosomal lipid metabolism and autophagy by TBI is an important contributor to subsequent development of AD/ADRD.